Storage materials which undergo a change in phase have been employed as an attractive alternative to materials which store thermal energy as sensible heat such as, for example, by raising the temperature of water or rocks. In contrast, a phase material (PCM) absorbs a large quantity of latent thermal energy during their phase change from a solid to a liquid and releases such thermal energy at a constant temperature as the process is reversed.
Several classes of phase change materials consisting of hydrated inorganic salts exist and can be generally divided into two groups:
(1) salt hydrates that go through many cycles of freezing and thawing without an appreciable separation of their chemical components are classified as congruent melting and eutectic salt hydrates, and
(2) salt hydrates which undergo an appreciable segregation of their chemical components during repeated cycles of freezing and thawing are generally classified as semi-congruent melting or incongruent melting.
Salt hydrates of group (1) which are "congruent melting" are hydrated materials such as, for example, CaBr.sub.2.6H.sub.2 O for which, at the melting point, with solid and liquid phases in stable equilibrium, the solid phase contains no hydrated CaBr.sub.2 other than the hexahydrte and the liquid phase contains 6 moles of water for every mole of CaBr.sub.2, plus sufficient water to form the stable hydrate of any additive materials in solution.
Eutectic salt hydrates are mixtures of two or more components mixed in such a ratio that the melting point of the mixture is lower than that of any component and the entire mixture at one and the same temperature passes from a solid form (frozen) into a liquid form and vice versa. An Example of a eutectic salt hydrate is one comprising 58.7 weight percent Mg(NO.sub.3).sub.2.6H.sub.2 O and 41.3 weight percent MgCl.sub.2.6H.sub.2 O . The salt hydrate of the type herein under consideration is a eutectic salt hydrate.
Mixtures of components which form a eutectic other than the eutectic composition hereinbefore defined are included within the scope of the present invention and are classified as either "hyper-eutectic" or "hypo-eutectic" hydrated salt mixtures. The hyper-eutectics contain more and the hypo-eutectics contain less of the principle component than the eutectic composition.
CaBr.sub.2.6H.sub.2 O contains 64.90 weight percent CaBr.sub.2 and melts at a temperature of 34.3.degree. C. The hexahydrate is congruent-melting, and forms a eutectic with the tetrahydrate containing 66.95 weight percent CaBr.sub.2 which melts at a temperature of 33.8.degree. C., with a 5.57 degree of hydration. While the heat of fusion for CaBr.sub.2.6H.sub.2 O is lower than for some other salt hydrates, a significant property is its very high density. On a volumetric basis, the heat of fusion of 54 cal/cm.sup.3 is high relative to many other hydrated inorganic salts. On a weight basis, the heat of fusion for CaBr.sub.2.6H.sub.2 O is 27.6 cal/g. For applications where space is at a premium, CaBr.sub.2.6H.sub.2 O is thus a very desirable phase change material.